1. Field of the Invention
The present invention relates to a light scanning device that deflects and scans a plurality of light beams on a photosensitive member e.g. of a laser printer, and an image forming apparatus that performs image formation using the light scanning device.
2. Description of the Related Art
In general, electrophotographic image forming apparatuses include e.g. a laser printer that performs image formation by scanning laser light on a photosensitive drum. In such a laser printer, a scanning optical system is used in which laser light emitted from a laser emitting device is collected by a lens system, and is deflected and scanned by a polygon mirror along with the rotation of a scanner motor.
In this laser printer, to adapt the printer to faster printing speed and higher resolution, there has been proposed a technique of performing image formation by increasing the number of laser emitting devices and simultaneously scanning a plurality of laser light beams. For example, when a surface emitting laser (hereinafter also referred to as VCSEL: vertical cavity surface emitting laser) is used, it is easy to form emission spots into an array and it is possible to configure the array such that a lot of emission spots are arranged on one chip.
By the way, some laser printers perform automatic light amount control (hereinafter also referred to as APC: automatic power control) in order to maintain the light amount of a laser light beam on a scanning surface at a constant level during image formation. In one method of APC, a laser is lit for a predetermined time period and a light amount of emitted laser light is detected by a light amount-detecting unit (PD: photodiode) which is internally or externally provided for the laser, for performing light detection, whereby current for driving the laser is feedback-controlled according the detected light amount.
In this method of APC, the photodiode detects laser light (rear light) emitted from an edge (end face) of a laser emitting device opposite from a laser emitting edge thereof. Timing of execution of APC is during scanning of the laser light beam through a non-image area outside an image area on a photosensitive member. This makes it possible to perform APC without affecting the image formation.
Further, in a scanning optical system that scans a plurality of beams, it is difficult to have the same number of photodiodes as the number of laser elements in association with the respective beams because of difficulty of arrangement thereof. Even if the same number of photodiodes can be arranged, this increases costs. To cope with this inconvenience, some scanning optical systems that scan a plurality of beams are configured such that a plurality of beams are received by a single photodiode, and a plurality of laser elements are sequentially lit for execution of APC, whereby the light amount is controlled on a laser element-by-laser element basis.
In the case of the VCSEL or the like of which the number of laser emission spots can be easily increased, however, the direction of emitting laser light beams is perpendicular to a semiconductor substrate, and hence it is difficult to arrange the photodiodes within the same package as in a semiconductor laser that emits light from its edge (end face).
To overcome this problem, in the case of the VCSEL or the like, there has been proposed a method in which light output from the front surface is separated by a half mirror to cause the same to enter a photodiode, whereby APC is performed using the single photodiode (see e.g. Japanese Patent Laid-Open Publication No. 2002-40350).
This method separates a light flux passed through an aperture which narrows down collimated light from a collimator lens that collimates light emitted from a surface emitting laser, into light going to a photosensitive member and light going to a photodiode, and performs APC using the single photodiode.
However, when APC is performed for a laser emitting device, such as a VCSEL, from which rear light cannot be detected, there arises the following problem:
When a VCSEL is used as a laser emitting device, APC cannot be performed without detecting light (front light) going to a photosensitive drum. For this reason, when the VCSEL is used, the amount of front light is adjusted by progressively increasing the same up to an amount at which a synchronization signal can be generated, thereafter, a non-image area is detected using the synchronization signal, and APC is performed in the detected non-image area.
However, in the VCSEL, the light amount is varied by influence of temperature on drive current, as shown in FIG. 3. Therefore, if the VCSEL has temperature characteristics as indicated by T2 in FIG. 3, it takes a long time before the amount of front light is adjusted to the amount at which the synchronization signal can be generated, by progressively increasing the value of current from a bias current value.